1. Field of the Invention
The present invention relates to a common mode choke coil for blocking a common mode noise current that flows in one of an input terminal and an output terminal and passes through the other terminal over a wide frequency band.
2. Description of the Related Art
The basic circuit structure of a general noise filter for attenuating a common mode noise (common mode noise current) is made up of a common mode choke coil in which an insulated copper wire is wound in same phase on a closed magnetic circuit core (a ferrite core is frequently used), an inter-line capacitor (called also xe2x80x9cX-capacitorxe2x80x9d) for removing a differential mode noise, and a line bypass capacitor (called also xe2x80x9cY-capacitorxe2x80x9d) for attenuating a common mode noise over a wide band.
The frequency band whose terminal noise to be regulated widely ranges from 10 kHz to 30 MHz, and its attenuating method is that the high frequency band components of the common mode noise are attenuated by a filter circuit essentially consisting of a common mode choke coil and a bypass capacitor, and the low frequency band components of the common mode noise are attenuated by making the impedance of the common mode choke coil high.
As a performance required for the common mode choke coil, it is desirable that the impedance is as large as possible to at least the frequency components of 10 kHz to 10 MHz (10,000 kHz) of the common mode noise. That is, there is required that the inductance is as large as possible in such a wide frequency range.
Up to now, as the closed magnetic circuit core, there are many cases in which the ferrite core is employed from the viewpoints of the costs and mass-production. Also, as the configuration of the magnetic core, the toroidal magnetic core is frequently employed. As other configurations, EE type, EI type, UU type, two-square-combined type or square-type magnetic core may be also used.
The common mode choke coil may be connected directly to the commercial AC line for use. In this event, consideration must be made on the safety from the viewpoint of withstanding voltage. Also, the common mode choke coil is used in the case where the common mode choke coil is inserted into a large-current circuit, since a bold wire must be wound on the magnetic core taking an obstacle caused by a rise in the temperature of the winding into account, the configuration is restricted by itself.
As a conventional example of the above-described common mode choke coil, a common mode choke coil 1 is shown in FIGS. 5 and 6.
Referring to FIG. 5, reference numeral 2 denotes a ring-shaped closed magnetic circuit core, and the closed magnetic circuit core 2 is inserted into an insulating case 3 made of plastics. The insulating case 3 is made up of a first case 4 and a second case 5 which are superimposed together. As shown in FIG. 6, two coils 6 and 7 are wound on the insulating case 3 into which the closed magnetic circuit core 2 is inserted (and then the closed magnetic circuit core 2 through the insulating case 3) in such a manner that magnetic fluxes developed by those two coils 6 and 7 cancel each other. In the present specification, those two coils 6 and 7 are called xe2x80x9cfirst coil 6xe2x80x9d and xe2x80x9csecond coil 7xe2x80x9d, respectively, for convenience.
Incidentally, in order to increase the attenuation amount of the common mode noise at the frequency band from 10 kHz to 10 MHz, the common mode choke coil has been required to increase the impedance of the common mode choke coil with respect to the common mode noise. That is, there has been required that the impedance of the winding increases at the frequency band from 10 kHz to 10 MHz.
To satisfy the above requirement, there have been countermeasures in which the coils usually employed are increased in size, two coils are connected in series, and so on. However, those countermeasures have not always been excellent because those may accompany an increase in costs, an increase in the number of parts to be used, an increase in the mounted space, etc.
Also, in order to increase the impedance particularly at a low frequency band, the number of turns of the coil is increased as large as possible, or a high permeability material is used as the magnetic core. However, in this case, the common mode choke coil has the characteristic that the increased number of turns leads to an increase in-line distributed capacity and the characteristic that the deterioration of the permeability is larger in the high frequency as the magnetic material is higher in permeability, as a result of which the deterioration of the impedance in the high frequency band becomes remarkable.
Also, in order to improve the frequency characteristic of the impedance in a wide band, there has been proposed a structure in which two kinds of magnetic cores different in frequency characteristic are disposed in parallel and then a coil is wound on those magnetic cores, such that a carbonyl iron dust core and a ferrite core are together as disclosed in, for example, Japanese Utility Model Laid-Open Publication No. Hei 4-32513, and that a magnetic core on which an amorphous magnetic material is wound and a dust core are combined together as disclosed in Japanese Utility Model Laid-Open Publication No. Sho 62-197823.
However, the structures disclosed in the above publications cannot sufficiently cover a wide frequency band of from 10 kHz to 10 MHz (that is, they cannot sufficiently increase the impedance so that the noise can be sufficiently attenuated in a wide frequency band from 10 kHz to 10 MHz).
On the other hand, in recent years, various electronic devices have progressively been downsized, reduced in the number of parts, saved in resource, saved in power and reduced in costs, and a common mode choke coil which is downsized and small in the number of turns as compared with the conventional one and can cover a wide frequency band by one choke coil has been demanded.
The present invention has been made in view of the above circumstances, and therefore an object of the present invention is to provide a common mode choke coil which is small in size and the number of turns and which can cover a wide frequency band by one choke coil.
In order to achieve the above object, according to a first aspect of the present invention, there is provided a common mode choke coil having two coils which are wound on a closed magnetic circuit core such that magnetic fluxes of the two coils cancel each other, characterized in that the closed magnetic circuit core is configured such that the closed magnetic circuit core is divided in parallel with the magnetic circuit thereof, and a plurality of divided magnetic cores which are formed of oxide magnetic substance made of the same material or the same kind of material are superimposed together for integration.
According to a second aspect of the invention, there is provided a common mode choke coil as defined in the first aspect, characterized in that the common mode choke coil comprises an insulating case into which the closed magnetic circuit core is inserted, and the coils are wound on the closed magnetic circuit core through the insulating case.
According to a third aspect of the invention, there is provided a common mode choke coil as defined in the first or second aspect, characterized in that the plurality of divided magnetic cores are identical in configuration with each other.
According to a fourth aspect of the invention, there is provided a common mode choke coil as defined in any one of the first to third aspects, characterized in that a shock absorber is interposed in at least one of a space between the insulating case and the closed magnetic circuit core and a space between the divided magnetic cores which are superimposed together.
According to a fifth aspect of the invention, there is provided a common mode choke coil as defined in any one of the first to third aspects, characterized in that adhesives are coated on at least one of a portion between the insulating case and the closed magnetic circuit core and a portion between the divided magnetic cores which are superimposed together.
According to a sixth aspect of the invention, there is provided a common mode choke coil as defined in the fourth or fifth aspect, characterized in that a gap is formed in at least one of a portion between the insulating case and the closed magnetic circuit core and a portion between the adjacent divided magnetic cores.